To meet the demand for wireless data traffic having increased since deployment of 4G (4th-Generation) communication systems, efforts have been made to develop an improved 5G (5th-Generation) or pre-5G communication system. Therefore, the 5G or pre-5G communication system is also called a ‘Beyond 4G Network’ or a ‘Post LTE System’. The 5G communication system is considered to be implemented in higher frequency (mmWave) bands, e.g., 60 GHz bands, so as to accomplish higher data rates. To decrease propagation loss of the radio waves and increase the transmission distance, the beamforming, massive multiple-input multiple-output (MIMO), Full Dimensional MIMO (FD-MIMO), array antenna, an analog beam forming, large scale antenna techniques are discussed in 5G communication systems. In addition, in 5G communication systems, development for system network improvement is under way based on advanced small cells, cloud Radio Access Networks (RANs), ultra-dense networks, device-to-device (D2D) communication, wireless backhaul, moving network, cooperative communication, Coordinated Multi-Points (CoMP), reception-end interference cancellation and the like. In the 5G system, Hybrid FSK and QAM Modulation (FQAM) and sliding window superposition coding (SWSC) as an advanced coding modulation (ACM), and filter bank multi carrier (FBMC), non-orthogonal multiple access (NOMA), and sparse code multiple access (SCMA) as an advanced access technology have been developed.
The Internet, which is a human centered connectivity network where humans generate and consume information, is now evolving to the Internet of Things (IoT) where distributed entities, such as things, exchange and process information without human intervention. The Internet of Everything (IoE), which is a combination of the IoT technology and the Big Data processing technology through connection with a cloud server, has emerged. As technology elements, such as “sensing technology”, “wired/wireless communication and network infrastructure”, “service interface technology”, and “Security technology” have been demanded for IoT implementation, a sensor network, a Machine-to-Machine (M2M) communication, Machine Type Communication (MTC), and so forth have been recently researched. Such an IoT environment may provide intelligent Internet technology services that create a new value to human life by collecting and analyzing data generated among connected things. IoT may be applied to a variety of fields including smart home, smart building, smart city, smart car or connected cars, smart grid, health care, smart appliances and advanced medical services through convergence and combination between existing Information Technology (IT) and various industrial applications.
In line with this, various attempts have been made to apply 5G communication systems to IoT networks. For example, technologies such as a sensor network, Machine Type Communication (MTC), and Machine-to-Machine (M2M) communication may be implemented by beamforming, MIMO, and array antennas. Application of a cloud Radio Access Network (RAN) as the above-described Big Data processing technology may also be considered to be as an example of convergence between the 5G technology and the IoT technology.
The cellular industry which has been predominantly operating over licensed spectrum is considering the usage of operation on unlicensed band in order to meet the surging traffic demands. The unlicensed bands are typically dominated by Wi-Fi and other technologies. An unlicensed band is free to be used by any technology but is governed by few regulations (in most countries) like the requirement of “Listen Before Talk—LBT” which requires a transmitter to sense the channel for at least 20 us, and if the channel is found to be free i.e. not used by other devices, then the device is allowed to transmit. Further, the regulations allow for transmissions up to a maximum time limit and also provide means for giving fairness to the other devices/technologies.
In the legacy 3GPP Long Term Evolution (3GPP LTE) system, a concept of carrier aggregation is used in which multiple carriers can be allocated to a multi-carrier capable User Equipment (UE), in order to boost the data rates. In this technique, one carrier is referred to as the Primary carrier and the other carriers are referred to as the secondary carriers. Subframe boundaries on all the carriers are considered to be aligned. Further, scheduling can be self-carrier based or cross carrier based. In the self-carrier mode, the Physical Downlink Control Channel (PDCCH) for a secondary carrier is sent on the secondary carrier itself, whereas in the cross carrier mode, the resource allocation for all the secondary carriers is contained in the PDCCH that is sent on the primary carrier only.
However, when the unlicensed carrier is used as a secondary carrier, the frame structure of the LTE should be largely re-used. This results in a channel retention issue, as the channel is to be retained till the upcoming subframe boundary. Since the channel sensing for the unlicensed carrier can be done at any time, if the channel is found to be available for use, then the channel needs to be retained until the start of the symbol boundary. Otherwise, other users of the unlicensed band (like Wi-Fi nodes) can usurp the channel. It is to be noted that if channel sensing is performed just before the start of the subframe boundary (for a duration as required by regulations), it drastically impacts the channel availability probability, as non LTE-U users of the channel (like Wi-Fi) do not work on fixed frame structures, and can sense and start transmission at any time.
During the channel retention duration, active communication with the intended receiver(s) is not possible as the PDCCH can only be sent at the start of the subframe in the legacy system. Hence there is a need to minimize the channel retention period in order to increase the unlicensed channel usage.